1. Field of the Invention
The present invention relates to a dispersion compensator which is used to compensate for the chromatic dispersion of a transmission optical fiber used in wavelength division multiplexing (WDM) optical transmission, an optical fiber which can be used in this dispersion compensator, an optical transmission line including the dispersion compensator, and an optical communications system including the optical transmission line.
2. Related Background Art
(Reference 1) A dispersion compensator which compensates for the chromatic dispersion of transmission optical fibers in the wavelength band of 1530 to 1610 nm is disclosed in L. V. Jorgensen et al., Next Generation Dispersion Compensating Modules for 40 GBit/s, Proceedings of NFOEC2002, USA, September 2002, pp. 1171-1182.
In cases where the chromatic dispersion of a transmission optical fiber is compensated for by means of such a dispersion compensator, the chromatic dispersion profile (defined by the orthogonal coordinate system of the wavelength and chromatic dispersion) of the overall optical transmission line including the transmission optical fiber and dispersion compensator has a portion that bulges in the plus direction and a portion that bulges in the minus portion as seen from the direct axis of chromatic dispersion, so that two extreme values are adopted with respect to the wavelength.
Furthermore, in a WDM optical communications system, signal light in which a plurality of channels of different wavelengths are multiplexed is transmitted via the optical transmission line, thus allowing the high-speed transmission and reception of a large volume of information. In order to obtain high-quality signal transmission in such a WDM optical communications system, it is necessary that the absolute value of the cumulative chromatic dispersion of the signal transmission line extending from the optical transmitter to the optical receiver be small over the entire signal wavelength band.
Transmission optical fibers used in the transmission lines of the optical communications system have a predetermined amount of chromatic dispersion in the signal wavelength band. Accordingly, a dispersion compensator which compensates for the chromatic dispersion of such transmission optical fibers is inserted into the signal transmission line. Specifically, since common transmission optical fibers have a positive chromatic dispersion, a dispersion compensator which has a negative chromatic dispersion is utilized. For example, a dispersion compensating optical fiber can be used as such a dispersion compensator. Furthermore, in order to compensate for dispersion in a broad wavelength region, studies have been conducted in which compensation is made not only for the chromatic dispersion, but also for the dispersion slope. For example, in the optical communications system described in U.S. Pat. No. 6,393,188, a reduction in the absolute value of the cumulative chromatic dispersion over the entire signal wavelength band is achieved for the entire optical transmission line, including the optical transmission fiber and dispersion compensator (dispersion compensating optical fiber).